Knowledge of the mechanism of estrogen biosynthesis and of the enzyme or enzymes involved in it is critical to an understanding of this essential biotransformation and to its rational control in diseases where estrogen formation needs to be limited. We will study the aromatization of androgens from several directions. The nature of the three oxidative steps participating in the process will be explored using novel specifically labeled substrates which allow examination both of the initial two hydroxylations at C-19 and the terminal hydroxylation leading to aromatization. Experimental evidence supports the 2Beta carbon as the site of the final hydroxylation, but other observations cast doubt on that location. Experiments are proposed to resolve the difficulty and to define the nature of the terminal enzymatic step in estrogen biosynthesis. The nature of estrogen biosynthesis. The nature of estrogen biosynthesis will be studied in tissues other than the placenta including the ovaries, testes, brain and other non-gonadal sites. Evidence has already been obtained that the process is different in the ovaries and brain and new tools will be used to probe the mechanism of aromatization in these tissues and to study the role of pituitary hormones and other agents in its control. New affinity media and specific radiolabeled irreversible ligands will be used to purify and separate the placental aromatase complex into its catalytic components. Immunocytochemical and autoradiographic methods will be used to study the localization of the enzymes in the various sites or aromatization. Information obtained in these studies will be used in the modification of the already synthesized new high affinity inhibitors of estrogen biosynthesis to generate new categories of those agents which will be expected to exhibit high in vivo potency in diminishing estrogenic levels in diseases such as breast cancer and prostatic hyperplasia and in controlling reproductive processes.